linux-kernel - Re: [PATCH v2] pwm_backlight: Add device tree support for Low Threshold Brightness

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Date:	Fri, 21 Sep 2012 11:43:39 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	"Philip, Avinash" <avinashphilip@...com>
CC:	"grant.likely@...retlab.ca" <grant.likely@...retlab.ca>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"rob@...dley.net" <rob@...dley.net>,
	"rpurdie@...ys.net" <rpurdie@...ys.net>,
	"thierry.reding@...onic-design.de" <thierry.reding@...onic-design.de>,
	"broonie@...nsource.wolfsonmicro.com" 
	<broonie@...nsource.wolfsonmicro.com>,
	"shawn.guo@...aro.org" <shawn.guo@...aro.org>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Nori, Sekhar" <nsekhar@...com>,
	"Hebbar, Gururaja" <gururaja.hebbar@...com>
Subject: Re: [PATCH v2] pwm_backlight: Add device tree support for Low Threshold
 Brightness

On 09/21/2012 12:03 AM, Philip, Avinash wrote:
> Hi Stephen,
> 
> On Fri, Sep 21, 2012 at 10:46:45, Stephen Warren wrote:
>> On 09/20/2012 10:51 PM, Philip, Avinash wrote:
>>> Some backlights perform poorly when driven by a PWM with a short
>>> duty-cycle. For such devices, the low threshold can be used to specify a
>>> lower bound for the duty-cycle and should be chosen to exclude the
>>> problematic range.
>>>
>>> This patch adds support for an optional low-threshold-brightness
>>> property.
>>
>>> diff --git a/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt
>>
>>>  Optional properties:
>>>    - pwm-names: a list of names for the PWM devices specified in the
>>>                 "pwms" property (see PWM binding[0])
>>> +  - low-threshold-brightness: brightness threshold low level. Low threshold
>>> +    brightness set to value so that backlight present on low end of
>>> +    brightness.
>>
>> For my education, why not just specify values above this value in the
>> brightness-levels array; how do those two interact?
> 
> Please find details from 
> https://lkml.org/lkml/2012/7/18/284

Hmm. That still doesn't really explain what this property does.

I'm going to guess that if this property is present, and values in the
brightness-levels property get scaled between the
low-threshold-brightness and 255 instead of being used directly. But
then, in the email you linked to, what does "But brightness-levels won't
be uniformly divided" mean? Why would doing the calculation at run-time
be any better than simply putting the correct values into
brightness-levels in the first place?

Either way, the DT binding should explain exactly what this value is
used for, and how it affects the interpretation of values in
brightness-levels.
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